The History of Galadriel and Celeborn

[William Cloud Hicklin]

You're still a bit off the mark. A "Mary Sue" is not a goody-two-shoes, but a character the author has become excessively fond of and consequently makes too omnicompetent and perfect. They tend to be characterised not by not taking part in "action," but by being preposterously good at it. I think, for example, in later Discworld books Pratchett made Sam Vimes a bit of a Mary Sue. (See also, Yoda in the prequels.)